Acoustic wave device, high-frequency front end circuit, and communication device

ABSTRACT

An acoustic wave device includes a piezoelectric substrate and an IDT electrode provided on the piezoelectric substrate. In the IDT electrode, a central region, first and second low acoustic velocity regions and first and second high acoustic velocity regions are disposed in this order. A duty ratio in the first low acoustic velocity region of first electrode fingers and the second low acoustic velocity region of second electrode fingers is larger than a duty ratio in the central region. When acoustic velocity of a transversal bulk wave propagating in metal that is a main component of a main electrode layer is defined as v (m/s), v≤3299 m/s, and when a wave length defined by an electrode finger pitch of the IDT electrode is defined as λ, and a film thickness of the main electrode layer normalized by the wave length λ is defined as T, then T≥0.00018e 0.002V +0.014.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese PatentApplication No. 2017-006106 filed on Jan. 17, 2017 and is a ContinuationApplication of PCT Application No. PCT/JP2018/001021 filed on Jan. 16,2018. The entire contents of each application are hereby incorporatedherein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an acoustic wave device using a pistonmode, a high-frequency front end circuit, and a communication device.

2. Description of the Related Art

In order to suppress a spurious component, an acoustic wave device usinga piston mode has been proposed.

For example, Japanese Unexamined Patent Application Publication No.2013-518455 discloses an example of an acoustic wave device using apiston mode. In the acoustic wave device, a region in which a pluralityof first electrode fingers and a plurality of second electrode fingersof an IDT electrode overlap each other when viewed in an acoustic wavepropagation direction is an intersecting region. In the acoustic wavedevice described in Japanese Unexamined Patent Application PublicationNo. 2013-518455, the intersecting region includes a central regionlocated at a center in a direction in which the first and secondelectrode fingers extend, and first and second edge regions provided onboth sides of the central region in the direction in which the first andsecond electrode fingers extend.

Additionally, in Japanese Unexamined Patent Application Publication No.2013-518455, a method of forming a low acoustic velocity region bylayering a mass addition film made of a dielectric or metal on the firstand second electrode fingers in the first and second edge regions, and amethod of forming a low acoustic velocity region by making a duty ratioin the first and second edge regions larger than a duty ratio in thecentral region are disclosed. A region outside the low acoustic velocityregion is a high acoustic velocity region in which acoustic velocity ishigher than acoustic velocity in the central region. By disposing thecentral region, the low acoustic velocity region, and the high acousticvelocity region in this order, energy of an acoustic wave is confinedand the spurious components caused by high-order transverse modes aresuppressed.

Here, in a case of the method of forming the low acoustic velocityregion by layering the mass addition film made of a dielectric or metalon the first and second electrode fingers, there is a problem in that amanufacturing cost becomes high because steps of exposure and filmformation are required.

Thus, from a viewpoint of the manufacturing cost, the method of formingthe low acoustic velocity region by making the duty ratio in the firstand second edge regions larger than the duty ratio in the central regionis preferable. However, in the case of this method, there is a problemin that, depending on a material and a film thickness of a mainelectrode layer in the IDT electrode, it is difficult to increase adifference in acoustic velocity between the low acoustic velocity regionand the central region to a desired extent or more, and thus thespurious components cannot be suppressed in some cases.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide acoustic wavedevices, high-frequency front end circuits, and communication devices,that are each able to effectively reduce or prevent the spuriouscomponents caused by high-order transverse modes, in an acoustic wavedevice using a piston mode, in which a low acoustic velocity region isprovided by making a duty ratio in a first edge region and a second edgeregion larger than a duty ratio in a central region.

An acoustic wave device according to a preferred embodiment of thepresent invention includes a piezoelectric body, and an IDT electrodeprovided on the piezoelectric body and including a main electrode layer,in which the IDT electrode includes a first busbar and a second busbarfacing each other, a plurality of first electrode fingers first ends ofwhich are connected with the first busbar, and a plurality of secondelectrode fingers first ends of which are connected with the secondbusbar, the plurality of second electrode fingers are interdigitatedwith the plurality of first electrode fingers, and include anintersecting region that is a portion in which the plurality of firstelectrode fingers and the plurality of second electrode fingers overlapeach other in an acoustic wave propagation direction, and when adirection in which the plurality of first electrode fingers extend or adirection in which the plurality of second electrode fingers extend isdefined as a length direction, the intersecting region includes, acentral region located at a central portion of the first electrodefingers and the second electrode fingers in the length direction, afirst low acoustic wave region which is disposed on an outside of thecentral region on a side of the first busbar in the length direction andin which an acoustic velocity is lower than an acoustic velocity in thecentral region, and a second low acoustic velocity region which isdisposed on an outside of the central region on a side of the secondbusbar in the length direction and in which an acoustic velocity islower than the acoustic velocity in the central region, and a first highacoustic velocity region which is disposed on an outside of the firstlow acoustic velocity region on a side of the first busbar in the lengthdirection and in which an acoustic velocity is higher than the acousticvelocity in the central region, and a second high acoustic velocityregion which is disposed on an outside of the second low acousticvelocity region on a side of the second busbar in the length directionand in which an acoustic velocity is higher than the acoustic velocityin the central region are provided, and a duty ratio in the first lowacoustic velocity region and the second low acoustic velocity region isgreater than a duty ratio in the central region, and v≤3299 m/s issatisfied, where v (m/s) represents acoustic velocity of a transversalbulk wave propagating in metal that is a main component of the mainelectrode layer, and when λ represents a wave length defined by anelectrode finger pitch of the IDT electrode and T represents a filmthickness of the main electrode layer normalized by the wave length λ,Formula 1 below is satisfied:

T≥0.00018e ^(0.002v)+0.014  Formula 1.

In an acoustic wave device according to a preferred embodiment of thepresent invention, the IDT electrode includes a plurality of layersincluding the main electrode layer.

In an acoustic wave device according to a preferred embodiment of thepresent invention, the main electrode layer contains any one of Au, Pt,Ta, Cu, Ni, and Mo as a main component.

In an acoustic wave device according to a preferred embodiment of thepresent invention, in the IDT electrode, v≤2895 m/s is satisfied, andFormula 2 below is satisfied:

T≥0.000029e ^(0.0032v)+0.02  Formula 2.

In this case, a duty ratio in the central region of the IDT electrode isable to be further increased under a condition that the spuriouscomponents caused by high-order transverse modes are able to beeffectively reduced or prevented. Thus, electric resistance of the IDTelectrode is able to be reduced, and an insertion loss is able to bereduced.

In an acoustic wave device according to a preferred embodiment of thepresent invention, in the IDT electrode, v≤2491 m/s is satisfied, andFormula 3 below is satisfied:

T≥0.000038e ^(0.0035v)+0.025  Formula 3.

In this case, the duty ratio in the central region of the IDT electrodeis able to be further increased under the condition that the spuriouscomponents caused by high-order transverse modes are able to beeffectively reduced or prevented. Thus, the electric resistance of theIDT electrode is able to be reduced, and the insertion loss is able tobe reduced.

In an acoustic wave device according to a preferred embodiment of thepresent invention, in the IDT electrode, v≤2289 m/s is satisfied, andFormula 4 below is satisfied:

T≥0.000020e ^(0.0042v)+0.03  Formula 4.

In this case, the duty ratio in the central region of the IDT electrodeis able to be further increased under the condition that the spuriouscomponents caused by high-order transverse modes are able t0 beeffectively reduced or prevented. Thus, the electric resistance of theIDT electrode is able to be reduced, and the insertion loss is able tobe reduced.

In an acoustic wave device according to a preferred embodiment of thepresent invention, in the IDT electrode, v≤2087 m/s is satisfied, andFormula 5 below is satisfied:

T≥0.000017e ^(0.0048v)+0.033  Formula 5.

In this case, the duty ratio in the central region of the IDT electrodeis able to be further increased under the condition that the spuriouscomponents caused by high-order transverse modes are able to beeffectively reduced or prevented. Thus, the electric resistance of theIDT electrode is able to be reduced, and the insertion loss is able tobe reduced.

In an acoustic wave device according to a preferred embodiment of thepresent invention, the piezoelectric body is made of LiNbO₃, Eulerangles (φ, θ, Ψ) of the piezoelectric body are Euler angles (0°±5°, θ,0°±10°), θ in the Euler angles (φ, θ, ψ) of the piezoelectric bodysatisfies θ≥27°. The Euler angles (φ, θ, ψ) are (0°±5°,{−0.054/(T×r−0.044)+31.33}°±1.5°, 0°±10°) and T×r≤0.10λ holds, when aratio of a density ρ of a material of the main electrode layer to adensity ρ_(Pt) of Pt is defined as r=ρ/ρ_(Pt). In this case, it ispossible to further reduce or prevent the spurious components caused byan SH wave.

In an acoustic wave device according to a preferred embodiment of thepresent invention, the first busbar and the second busbar of the IDTelectrode include cavities, and in the first and second busbars, aportion located closer to the central region in the length directionthan the cavity is an inner busbar portion, a portion opposite to theinner busbar portion with the cavity interposed is an outer busbarportion, and in the first busbar, the inner busbar portion is a lowacoustic velocity region, a region in which the cavity is provided isthe first high acoustic velocity region, and in the second busbar, theinner busbar portion is a low acoustic velocity region, and a region inwhich the cavity is provided is the second high acoustic velocityregion.

An acoustic wave device according to a preferred embodiment of thepresent invention includes a piezoelectric body, and an IDT electrodeprovided on the piezoelectric body and including a main electrode layer,in which the IDT electrode includes a first busbar and a second busbarfacing each other, a plurality of first electrode fingers first ends ofwhich are connected with the first busbar, and a plurality of secondelectrode fingers first ends of which are connected with the secondbusbar, the plurality of second electrode fingers are interdigitatedwith the plurality of first electrode fingers, and includes anintersecting region in which the plurality of first electrode fingersand the plurality of second electrode fingers overlap each other in anacoustic wave propagation direction, and when a direction in which theplurality of first electrode fingers extend or a direction in which theplurality of second electrode fingers extend is defined as a lengthdirection, the intersecting region includes, a central region located ata central portion of the first electrode fingers and the secondelectrode fingers in the length direction, a first low acoustic waveregion which is disposed on an outside of the central region on a sideof the first busbar in the length direction and in which an acousticvelocity is lower than an acoustic velocity in the central region, and asecond low acoustic velocity region which is disposed on an outside ofthe central region on a side of the second busbar in the lengthdirection and in which an acoustic velocity is lower than the acousticvelocity in the central region, and a first high acoustic velocityregion which is disposed on an outside of the first low acousticvelocity region on a side of the first busbar in the length directionand in which an acoustic velocity is higher than the acoustic velocityin the central region, and a second high acoustic velocity region whichis disposed on an outside of the second low acoustic velocity region ona side of the second busbar in the length direction and in which anacoustic velocity is higher than the acoustic velocity in the centralregion are provided, and V2/V1≤0.98 is satisfied, where V1 representsacoustic velocity in the central region and V2 represents acousticvelocity in the first low acoustic velocity region and the second lowacoustic velocity region.

In an acoustic wave device according to a preferred embodiment of thepresent invention, a dielectric film is further provided on thepiezoelectric body so as to cover the IDT electrode. In this case, asurface of the IDT electrode is able to be protected, and the IDTelectrode is less likely to be damaged.

A high-frequency front end circuit according to a preferred embodimentof the present invention includes an acoustic wave device according to apreferred embodiment of the present invention and a power amplifier.

A communication device according to a preferred embodiment of thepresent invention includes a high-frequency front end circuit accordingto a preferred embodiment of the present invention and an RF signalprocessing circuit.

According to preferred embodiments of the present invention, acousticwave devices using a piston mode are provided in each of which a lowacoustic velocity region is provided by making a duty ratio in a firstedge region and a second edge region larger than a duty ratio in acentral region, high-frequency front end circuits, and communicationdevices that are able to effectively reduce or prevent the spuriouscomponents caused by high-order transverse modes.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an acoustic wave device according to a firstpreferred embodiment of the present invention.

FIG. 2 is an enlarged front sectional view of an IDT electrode in thefirst preferred embodiment according to the present invention.

FIG. 3 is a diagram illustrating a relationship between a normalizedoverlap integral value in a fundamental mode and an acoustic velocityratio V2/V1.

FIG. 4 is a diagram illustrating a relationship between a duty ratio anda normalized acoustic velocity when a thickness of a main electrodelayer made of Pt is set to about 0.02λ.

FIG. 5 is a diagram illustrating a relationship between the duty ratioand the normalized acoustic velocity when the thickness of the mainelectrode layer made of Pt is set to about 0.04λ.

FIG. 6 is a diagram illustrating a relationship between the duty ratioand the normalized acoustic velocity when the thickness of the mainelectrode layer made of Pt is set to about 0.06λ.

FIG. 7 is a diagram illustrating a relationship between a duty ratio ina central region at which the acoustic velocity ratio V2/V1 becomesabout 0.98 when the acoustic velocity V2 in a first low acousticvelocity region and a second low acoustic velocity region becomes thelowest, and the film thickness of the main electrode layer made of Pt.

FIG. 8 is a diagram illustrating a relationship between a maximum valueof the duty ratio in the central region at which the acoustic velocityratio V2/V1 becomes about 0.98 and an acoustic velocity v of atransversal bulk wave propagating in metal that is a main component ofthe main electrode layer.

FIG. 9 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer and a film thickness T ofthe main electrode layer at which the duty ratio in the central regionis about 0.40 and the acoustic velocity ratio V2/V1 becomes about 0.98.

FIG. 10 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.45 and the acoustic velocity ratio V2/V1 becomes about0.98.

FIG. 11 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.50 and the acoustic velocity ratio V2/V1 becomes about0.98.

FIG. 12 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.525 and the acoustic velocity ratio V2/V1 becomesabout 0.98.

FIG. 13 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.55 and the acoustic velocity ratio V2/V1 becomes about0.98.

FIG. 14 is an enlarged front sectional view of first electrode fingersand second electrode fingers of an IDT electrode in a first modificationexample of the first preferred embodiment according to the presentinvention.

FIG. 15 is an enlarged front sectional view of first electrode fingersand second electrode fingers of an IDT electrode in a secondmodification example of the first preferred embodiment according to thepresent invention.

FIG. 16 is an enlarged front sectional view of first electrode fingersand second electrode fingers of an IDT electrode in a third modificationexample of the first preferred embodiment according to the presentinvention.

FIG. 17 is an enlarged plan view illustrating a vicinity of a firstbusbar in a fourth modification example of the first preferredembodiment of the present invention.

FIG. 18 is a front sectional view of an acoustic wave device accordingto a fifth modification example of the first preferred embodimentaccording to the present invention.

FIG. 19 is a diagram illustrating impedance frequency characteristics ofan acoustic wave device according to a second preferred embodiment ofthe present invention.

FIG. 20 is a diagram illustrating a return loss of the acoustic wavedevice according to the second preferred embodiment of the presentinvention.

FIG. 21 is a diagram illustrating impedance frequency characteristics ofan acoustic wave device of a comparative example.

FIG. 22 is a diagram illustrating a return loss of the acoustic wavedevice of the comparative example.

FIG. 23 is a diagram illustrating a range of θ in Euler angles (φ, θ, ψ)of a piezoelectric substrate in the second preferred embodimentaccording to the present invention.

FIG. 24 is a block diagram of a communication device including ahigh-frequency front end circuit according to a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

The preferred embodiments described herein are illustrative and apartial substitution or combination of configurations may be possiblebetween different preferred embodiments.

FIG. 1 is a plan view of an acoustic wave device according to a firstpreferred embodiment of the present invention. In FIG. 1, first andsecond dielectric films that will be described later are omitted.

An acoustic wave device 1 is preferably, for example, a one-portacoustic wave resonator. The acoustic wave device 1 includes apiezoelectric substrate 2 defining a piezoelectric body. Thepiezoelectric substrate 2 is preferably made of, for example, LiNbO₃.

An IDT electrode 3 is provided on the piezoelectric substrate 2. Byapplying an AC voltage to the IDT electrode 3, an acoustic wave isexcited. A reflector 4 and a reflector 5 are disposed on both sides inan acoustic wave propagation direction of the IDT electrode 3,respectively.

The IDT electrode 3 includes a first busbar 3 a 1 and a second busbar 3b 1 opposed to each other. The IDT electrode 3 includes a plurality offirst electrode fingers 3 a 2 first ends of which are connected with thefirst busbar 3 a 1. Further, the IDT electrode 3 includes a plurality ofsecond electrode fingers 3 b 2 first ends of which are connected withthe second busbar 3 b 1.

The plurality of first electrode fingers 3 a 2 and the plurality ofsecond electrode fingers 3 b 2 are interdigitated with each other. TheIDT electrode 3 includes an intersecting region A where the firstelectrode fingers 3 a 2 and the second electrode fingers 3 b 2 overlapeach other in the acoustic wave propagation direction. Here, a directionin which the first electrode fingers 3 a 2 and the second electrodefingers 3 b 2 extend is defined as a length direction of the firstelectrode fingers 3 a 2 and the second electrode fingers 3 b 2. Withthis, the intersecting region A includes a central region A1 located ata central portion in the length direction, and a first edge region A2 aand a second edge region A2 b disposed on both sides of the centralregion A1 in the length direction respectively. The first edge region A2a is located on a side of the first busbar 3 a 1, and the second edgeregion A2 b is located on a side of the second busbar 3 b 1.

The IDT electrode 3 includes a first outer region Ba on a side of thefirst edge region A2 a opposite to a side of the central region A1 and asecond outer region Bb on a side of the second edge region A2 b oppositeto a side of the central region A1. The first outer region Ba is locatedbetween the first edge region A2 a and the first busbar 3 a 1. Thesecond outer region Bb is located between the second edge region A2 band the second busbar 3 b 1.

FIG. 2 is an enlarged front sectional view of the IDT electrode in thefirst preferred embodiment. Here, a dimension along the acoustic wavepropagation direction in the first electrode finger and the secondelectrode finger is defined as a width. A dimension “a” in FIG. 2indicates a width of the first electrode finger or the second electrodefinger. A dimension “b” indicates a distance between one end of thefirst electrode finger along the acoustic wave propagation direction andthe one end of the second electrode finger adjacent to the firstelectrode finger.

The IDT electrode 3 is preferably made of a layered metal film includinga plurality of metal layers that are laminated. The IDT electrode 3includes a main electrode layer 6 b. The main electrode layer 6 boccupies a largest mass among the plurality of metal layers of the IDTelectrode 3. In the present preferred embodiment, the main electrodelayer is preferably made of, for example, metal having density higherthan that of Al. As such metal, it is preferable to use, for example,one containing any one of Au, Pt, Ta, Cu, Ni, and Mo as a maincomponent. Note that, the material of the main electrode layer 6 b isnot limited to the above, and any metal having higher density than thatof Al is sufficient. Thus, even when a dielectric film is provided onthe IDT electrode 3, a reflection coefficient of an acoustic wave isable to be increased.

In the present preferred embodiment, in the IDT electrode 3, aconductive auxiliary electrode layer 6 d is provided on the mainelectrode layer 6 b provided on the piezoelectric substrate 2. Theconductive auxiliary electrode layer 6 d is preferably made of metalhaving lower electric resistance than that of the main electrode layer 6b. More specifically, the conductive auxiliary electrode layer 6 d ispreferably made of, for example, Al. Since the conductive auxiliaryelectrode layer 6 d is included, electric resistance of the IDTelectrode 3 is able to be reduced. Note that, layered structure of theIDT electrodes 3 is not limited to the above. Further, the IDT electrode3 may be made of a single-layer metal film including only the mainelectrode layer 6 b.

A first dielectric film 7 defining and functioning as a dielectric filmof the present invention is provided on the piezoelectric substrate 2 soas to cover the IDT electrode 3. In the present preferred embodiment,the first dielectric film 7 is preferably made of, for example, siliconoxide such as SiO₂. As a result, frequency temperature characteristicsare able to be improved. In addition, a surface of the IDT electrode 3is able to be protected, and the IDT electrode 3 is less likely to bedamaged. Note that, a material of the first dielectric film 7 is notlimited to the above. The first dielectric film 7 need not be provided.

A second dielectric film 8 is provided on the first dielectric film 7.In the present preferred embodiment, the second dielectric film 8 ispreferably made of, for example, silicon nitride such as SiN. Byadjusting a film thickness of the second dielectric film 8, it ispossible to easily adjust a frequency. Note that, a material of thesecond dielectric film 8 is not limited to the above. The seconddielectric film 8 need not be provided.

Referring back to FIG. 1, each of the plurality of first electrodefingers 3 a 2 includes wide portions 3 a 3 having a width larger thanthat of other portions in the first edge region A2 a and the second edgeregion A2 b, respectively. Similarly, each of the plurality of secondelectrode fingers 3 b 2 includes wide portions 3 b 3.

Since the plurality of first electrode fingers 3 a 2 include the wideportions 3 a 3, and the plurality of second electrode fingers 3 b 2include the wide portions 3 b 3, an acoustic velocity of acoustic wavesin the first edge region A2 a and the second edge region A2 b is lowerthan a propagation velocity (hereinafter, referred to as acousticvelocity) in a propagation direction of an acoustic wave in the centralregion A1. Here, an acoustic velocity of the acoustic wave in thecentral region A1 is defined as V1, and an acoustic velocity of theacoustic wave in the first edge region A2 a and the second edge regionA2 b is defined as V2. With this, V1>V2 is satisfied. In this manner,the first edge region A2 a is defined as a first low acoustic velocityregion, and the second edge region A2 b is defined as a second lowacoustic velocity region.

A portion located in the first outer region Ba includes only the firstelectrode fingers 3 a 2. A portion located in the second outer region Bbincludes only the second electrode fingers 3 b 2. Thus, the acousticvelocity of acoustic waves in the first outer region Ba and the secondouter region Bb is higher than the acoustic velocity of the acousticwave in the central region A1. Here, the acoustic velocity of theacoustic waves in the first outer region Ba and the second outer regionBb is defined as V3. With this, V3>V1 is satisfied. As described above,the first outer region Ba and the second outer region Bb are a firsthigh acoustic velocity region and a second high acoustic velocityregion, respectively, in which acoustic velocity is higher than that inthe central region A1.

The first and second low acoustic velocity regions are disposed outsidethe central region A1, and the first and second high acoustic velocityregions are disposed outside the first and second low acoustic velocityregions, respectively. Here, a dimension of the first and second lowacoustic velocity regions along a direction orthogonal or substantiallyorthogonal to the acoustic wave propagation direction is defined as awidth of the first and second low acoustic velocity regions. Byadjusting the acoustic velocity V1 in the central region A1, theacoustic velocity V2 in the first and second low acoustic velocityregions, the acoustic velocity V3 in the first and second high acousticvelocity regions, and the width of the first and second low acousticvelocity regions, it is possible to make an acoustic wave displacementdistribution in a direction in which the first electrode fingers extendand in a direction in which the second electrode fingers extend in thecentral region A1 constant or substantially constant. As a result, apiston mode is generated, thus the spurious components caused byhigh-order transverse modes are able to be reduced or prevented. Asdescribed above, the acoustic wave device 1 utilizes the piston mode.

The above-described relationship among the acoustic velocities V1, V2and V3 is illustrated in FIG. 1. It is illustrated that in FIG. 1, theacoustic velocity on the outer side is higher.

Here, the inventors of preferred embodiments of the present inventionhave discovered that, in the acoustic wave device 1 using the pistonmode, when the acoustic velocity ratio V2/V1 becomes about 0.98 or less,the spurious components are able to be effectively reduced or prevented.

Additionally, the inventors of preferred embodiments of the presentinvention have also discovered that in the acoustic wave device 1 usingthe piston mode (hereinafter, referred to as “plane piston mode”) inwhich the first low acoustic velocity region and the second low acousticvelocity region are provided by making a duty ratio in the first edgeregion A2 a and a duty ratio in the second edge region A2 b larger thana duty ratio in the central region A1, conditions 1) and 2) under whichthe acoustic velocity ratio V2/V1 becomes about 0.98 or less aredescribed below.

Note that, the duty ratio is a percentage of a portion where theelectrode is provided in the acoustic wave propagation direction, and isexpressed by a/b by using the dimension “a” and the dimension “b” inFIG. 2. Further, it is known to those skilled in the art that when theduty ratio is excessively small, the IDT electrode cannot be provided,and when the duty ratio is excessively large, a gap between theelectrode fingers of the IDT electrode cannot be provided and a shortcircuit occurs, thus it is difficult to provide the IDT electrode whenthe duty ratio is not within a range of about 0.30 or more and about0.80 or less.

1) v≤3299 m/s, where v (m/s) is acoustic velocity of a transversal bulkwave propagating in metal that is a main component of the main electrodelayer 6 b.

2) The following Formula 1 is satisfied when λ is a wave length definedby an electrode finger pitch of the IDT electrode and T is a filmthickness of the main electrode layer 6 b normalized by the wave lengthλ:

T≥0.00018e ^(0.002v)+0.014  Formula 1.

That is, the inventors of preferred embodiments of the present inventionhave discovered that, in the acoustic wave device 1 using the planepiston mode, by selecting a material and a film thickness of the mainelectrode layer 6 b in the IDT electrode 3 that satisfy theconditions 1) and 2), an acoustic velocity difference between the firstlow acoustic velocity region and the second low acoustic velocityregion, and the central region is able to be increased to a certainextent or more, and the spurious components are able to be reduced orprevented.

As described above, in the acoustic wave device 1 using the piston mode,when the acoustic velocity ratio V2/V1 becomes about 0.98 or less, thespurious components are able to be effectively reduced or prevented.This is illustrated in FIG. 3.

FIG. 3 is a diagram illustrating a relationship between a normalizedoverlap integral value in a fundamental mode and the acoustic velocityratio V2/V1. A vertical axis in FIG. 3 indicates the normalized overlapintegral value in the fundamental mode described in Japanese UnexaminedPatent Application Publication No. 2013-518455. This integral value isused as an index indicating a degree to which the spurious componentscaused by high-order transverse modes are able to be reduced orprevented, and indicates that as a value of the normalized overlapintegral value in the fundamental mode is closer to 1, the spuriouscomponents caused by high-order transverse modes are reduced orprevented. A horizontal axis indicates the acoustic velocity ratio V2/V1of the first low acoustic velocity region and the second low acousticvelocity region, and the central region.

In order to determine the relationship illustrated in FIG. 3, thefollowing conditions were used. Note that, a dimension along a directionorthogonal or substantially orthogonal to an acoustic wave propagationdirection in the intersecting region is defined as an intersectingwidth. The direction orthogonal or substantially orthogonal to theacoustic wave propagation direction is defined as an intersecting widthdirection.

Intersecting width: about 10λ

An acoustic velocity ratio V3/V1 between the first high acousticvelocity region and the second high acoustic velocity region, and thecentral region: about 1.08

An anisotropy coefficient (1+Γ in the following formula): about 0.7485

The width of the first low acoustic velocity region and second lowacoustic velocity region: set according to the following Formula 6.

Note that, the following Formula 6 is represented by an Expression 5 inJapanese Unexamined Patent Application Publication No. 2013-518455(corresponding International Publication No. is WO2011/088904).

$\begin{matrix}{\mspace{79mu} {{Expression}\mspace{14mu} 1}} & \; \\{\mspace{79mu} {{W = {\frac{\text{?}}{2\; \pi \; f}{\sqrt{1 + \Gamma}}\frac{\arctan \sqrt{\text{?}}}{\sqrt{2\text{?}}}}}{\text{?}\text{indicates text missing or illegible when filed}}}} & {{Formula}\mspace{14mu} 6}\end{matrix}$

Note that, Japanese Unexamined Patent Application Publication No.2013-518455 describes that, when a width of a first low acousticvelocity region and a second low acoustic velocity region is setaccording to the Formula 6, displacement distribution of acoustic wavesin a central region is able to be made constant or substantiallyconstant, and a piston mode is generated.

Under this condition, a change in the displacement distribution in theintersecting width direction in the fundamental mode was determined whenthe acoustic velocity ratio V2/V1 of the first low acoustic velocityregion and the second low acoustic velocity region, and the centralregion was changed. The relationship in FIG. 3 shows normalized overlapintegral values determined by using the above.

As illustrated in FIG. 3, under the conditions in which the acousticvelocity ratio V2/V1 is about 0.98 or less, the overlap integral valuesare constant or approximately constant at a value of about 0.992 that isa value close to 1. However, when the acoustic velocity ratio V2/V1becomes larger than about 0.98, the overlap integral value decreasesrapidly. Thus, under the conditions in which the acoustic velocity ratioV2/V1 is about 0.98 or less, it is possible to effectively reduce orprevent the spurious components caused by high-order transverse modes.

Note that, in addition to the plane piston mode, also in an acousticwave device using another common piston mode, when the acoustic velocityratio V2/V1 becomes about 0.98 or less, the spurious components are ableto be effectively reduced or prevented.

Next, it is illustrated below that one of conditions under which theacoustic velocity ratio V2/V1 becomes about 0.98 or less in the acousticwave device using the plane piston mode may be manufactured is thefollowing 1).

1) v≤3299 m/s, where v (m/s) is the acoustic velocity of the transversalbulk wave propagating in the metal that is the main component of themain electrode layer 6 b.

Here, in order to reduce or minimize the acoustic velocity ratio V2/V1as far as possible during the manufacturing, it is necessary to set theminimum value of acoustic velocity as far as possible in themanufacturing to V2, and to set the maximum value of acoustic velocityas far as possible during the manufacturing to V1.

When Pt is used for the main electrode layer and the film thickness ofthe main electrode layer is set to about 0.02λ, about 0.04λ, and about0.06λ, changes in acoustic velocity of an acoustic wave with respect tochanges in duty ratio was determined. Conditions are as follows.

Piezoelectric substrate: material LiNbO₃, Euler angles (0°, 30°, 0°)

Main electrode layer: material Pt, film thickness about 0.02λ, about0.04λ, about 0.06λ

Conductive auxiliary electrode layer: material Al, film thickness about0.08λ

First dielectric film: material SiO₂, film thickness about 0.30λ

Second dielectric film: material SiN, film thickness about 0.01λ

Acoustic wave used: Rayleigh wave

FIG. 4 is a diagram illustrating a relationship between the duty ratioand the normalized acoustic velocity when the thickness of the mainelectrode layer made of Pt is set to about 0.02λ. FIG. 5 is a diagramillustrating a relationship between the duty ratio and the normalizedacoustic velocity when the thickness of the main electrode layer made ofPt is set to about 0.04λ. FIG. 6 is a diagram illustrating arelationship between the duty ratio and the normalized acoustic velocitywhen the thickness of the main electrode layer made of Pt is set toabout 0.06λ. Note that, in each of FIG. 4 to FIG. 6, a vertical axisrepresents the normalized acoustic velocity normalized, with acousticwave velocity when the duty ratio is about 0.5 being 1.

As illustrated in FIG. 4 to FIG. 6, when Pt is used for the mainelectrode layer, the acoustic velocity of the acoustic wave becomeshigher as the duty ratio becomes smaller, and the acoustic velocity ofthe acoustic wave becomes lower as the duty ratio becomes larger. Thus,by making a width in the first edge region larger than a width in thecentral region of the first electrode fingers and the second electrodefingers, it is possible to configure the first low acoustic velocityregion. Similarly, by making a width in the second edge region largerthan the width in the central region of the first electrode fingers andthe second electrode fingers, it is possible to configure the second lowacoustic velocity region.

Further, since the acoustic velocity increases as the duty ratio isreduced, the acoustic velocity V1 in the central region is able to beincreased as the duty ratio in the central region is reduced, and theacoustic velocity ratio V2/V1 is able to be reduced. Thus, the spuriouscomponents caused by high-order transverse modes are able to beeffectively reduced or prevented.

Note that, when the duty ratio is made excessively small, the width ofthe first electrode finger and the second electrode finger becomesexcessively narrow, so that it becomes difficult to provide the IDTelectrode stably, and a problem arises in that manufacturing becomesdifficult. Thus, it is preferable to set the minimum value of the dutyratio to about 0.30 or more, for example.

As illustrated in FIG. 4 to FIG. 6, the change in the normalizedacoustic velocity with respect to the change in the duty ratio is in adownward convex state. That is, inclination of a curve indicating thechange in the normalized acoustic velocity becomes gentler as the dutyratio increases. The normalized acoustic velocity is reduced orminimized in a vicinity of the duty ratio of about 0.80. Thus, it ispreferable that the duty ratio in the first low acoustic velocity regionand the second low acoustic velocity region is set to a value at whichthe acoustic velocity is minimized. However, when the duty ratio isexcessively increased, an insulation withstand voltage between the firstelectrode fingers and the second electrode fingers adjacent to eachother deteriorates, making manufacturing difficult, and thus, it ispreferable to set the duty ratio to about 0.80 or less, for example.When Pt is used for the main electrode layer, the duty ratio in thefirst low acoustic velocity region and the second low acoustic velocityregion is preferably about 0.80, for example.

When the film thickness of the main electrode layer made of Pt is about0.02λ and the duty ratio is about 0.80, the normalized acoustic velocitybecomes about 0.989 and is reduced or minimized. That is, when thiscondition is selected for the duty ratio in the first low acousticvelocity region and the second low acoustic velocity region, thenormalized acoustic velocity V2 in the first low acoustic velocityregion and the second low acoustic velocity region becomes about 0.989.Note that, the normalized acoustic velocity in the first low acousticvelocity region and the second low acoustic velocity region is denotedby V2, similar to the acoustic velocity. The normalized acousticvelocity in the central region is also denoted by V1, similar to theacoustic velocity.

On the other hand, when the duty ratio in the central region is set toabout 0.41, the normalized acoustic velocity V1 in the central regionbecomes about 1.009. At this time, the acoustic velocity ratio of thefirst low acoustic velocity region and the second low acoustic velocityregion to the central region satisfies V2/V1=0.989/1.009=about 0.98.Thus, when the film thickness of the main electrode layer made of Pt isabout 0.02λ, by setting the duty ratio in the central region to about0.41 or less, the acoustic velocity ratio V2/V1 of the first lowacoustic velocity region and the second low acoustic velocity region,and the central region is able to be set to about 0.98 or less.

Similarly, when the film thickness of the main electrode layer is about0.04λ, and the duty ratio is about 0.80, the normalized acousticvelocity is minimized at about 0.980. That is, when this condition isselected for the duty ratio in the first low acoustic velocity regionand the second low acoustic velocity region, the normalized acousticvelocity V2 in the first low acoustic velocity region and the second lowacoustic velocity region becomes about 0.980. On the other hand, whenthe duty ratio in the central region is set to about 0.50, thenormalized acoustic velocity V1 in the central region becomes about1.000. At this time, the acoustic velocity ratio of the first lowacoustic velocity region and the second low acoustic velocity region tothe central region satisfies V2/V1=0.980/1.000=about 0.98. Thus, whenthe film thickness of the main electrode layer made of Pt is about0.04λ, by setting the duty ratio in the central region to about 0.50 orless, the acoustic velocity ratio V2/V1 of the first low acousticvelocity region and the second low acoustic velocity region, and thecentral region is able to be set to about 0.98 or less.

Similarly, when the film thickness of the main electrode layer is about0.06λ and the duty ratio is about 0.80, the normalized acoustic velocityis minimized at about 0.974. That is, when this condition is selectedfor the duty ratio in the first low acoustic velocity region and thesecond low acoustic velocity region, the normalized acoustic velocity V2in the first low acoustic velocity region and the second low acousticvelocity region becomes about 0.974. On the other hand, when the dutyratio in the central region is set to about 0.53, the normalizedacoustic velocity V1 in the central region becomes about 0.994. At thistime, the acoustic velocity ratio of the first low acoustic velocityregion and the second low acoustic velocity region to the central regionsatisfies V2/V1=0.974/0.994=about 0.98. Thus, when the film thickness ofthe main electrode layer made of Pt is about 0.06λ, by setting the dutyratio in the central region to about 0.53 or less, the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region can be set to about0.98 or less.

When the film thickness of the main electrode layer made of Pt was otherthan the above, the duty ratio in the central region at which theacoustic velocity ratio V2/V1 became about 0.98 was determined, when theduty ratio in the first low acoustic velocity region and the second lowacoustic velocity region was set to a duty ratio at which the acousticvelocity V2 was the lowest as far as possible during the manufacturing.This is illustrated in FIG. 7 below.

FIG. 7 is a diagram illustrating a relationship between the duty ratioin the central region at which the acoustic velocity ratio V2/V1 becomesabout 0.98 when the acoustic velocity V2 in the first low acousticvelocity region and the second low acoustic velocity region becomes thelowest, and the film thickness of the main electrode layer made of Pt.

As illustrated in FIG. 7, in order to increase the duty ratio in thecentral region in a state where the acoustic velocity ratio V2/V1 isabout 0.98, it is necessary to increase the film thickness of the mainelectrode layer. However, even when the film thickness of the mainelectrode layer is set to about 0.12λ or more, a duty ratio at which theacoustic velocity ratio V2/V1 is able to be set to about 0.98 does notexceed about 0.557. That is, in order to achieve the plane piston modeby using Pt for the main electrode layer, it is necessary to set theduty ratio in the central region to about 0.557 or less, and further toset the film thickness of the main electrode layer according to thatduty ratio. More specifically, it is necessary to set the film thicknessof the main electrode layer to a film thickness such that a value of theduty ratio in the central region is equal to or larger than a valueindicated by the curve in FIG. 7.

In the conventional knowledge, it has been considered that by increasinga film thickness of a main electrode layer, the dependency of acousticvelocity on a duty ratio is increased, and an acoustic velocitydifference is able to be easily increased. However, surprisingly, theinventors of preferred embodiments of the present invention havediscovered from the above results that the dependency of the acousticvelocity on the duty ratio does not increase even when the filmthickness of the main electrode layer is increased to a certain extentor more. That is, the inventors of preferred embodiments of the presentinvention have discovered that there is an upper limit to the duty ratioin the central region at which the spurious components are able to bereduced or prevented by the plane piston mode.

Similar consideration was given to a case where metal other than Pt wasused for the main electrode layer, and a maximum duty ratio in thecentral region at which the acoustic velocity ratio V2/V1 is able to beset to about 0.98 was determined. This was done for a plurality of typesof metal.

Then, a relationship between the maximum duty ratio in the centralregion determined in the above at which the acoustic velocity ratioV2/V1 is able to be set to about 0.98, and the acoustic velocity v ofthe transversal bulk wave propagating in the main electrode layer wasdetermined.

The results are illustrated in FIG. 8. Note that, FIG. 8 is a diagramillustrating the relationship between the maximum value of the dutyratio in the central region at which the acoustic velocity ratio V2/V1becomes about 0.98 and the acoustic velocity v of the transversal bulkwave propagating in metal that is a main component of the main electrodelayer.

The acoustic velocity of the transversal bulk wave propagating in themetal is an inherent value in each of various metals. Here, the acousticvelocity of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer is defined as v (m/s).The acoustic velocity v of the transversal bulk wave propagating in themetal is expressed by v=(c₄₄/ρ)^(0.5). Note that, ρ (kg/m³) is thedensity of the metal, and c₄₄ (Pa) is one of elements of an elasticstiffness constant of the metal. Since the metal handled here may beregarded as an isotropic body, an elastic stiffness constant c_(ij) isexpressed by the following determinant.

$\begin{matrix}{{{{c\text{?}} = \begin{bmatrix}{c_{12} + {2c_{44}}} & c_{12} & c_{12} & 0 & 0 & 0 \\c_{12} & {c_{12} + {2c_{44}}} & c_{12} & 0 & 0 & 0 \\c_{12} & c_{12} & {c_{12} + {2c_{44}}} & 0 & 0 & 0 \\0 & 0 & 0 & c_{44} & 0 & 0 \\0 & 0 & 0 & 0 & c_{44} & 0 \\0 & 0 & 0 & 0 & 0 & c_{44}\end{bmatrix}}{\text{?}\text{indicates text missing or illegible when filed}}}\mspace{205mu}} & {{Expression}\mspace{14mu} 2}\end{matrix}$

Here, the density ρ (kg/m³), the elastic constant c₄₄, and the acousticvelocity v of the transversal bulk wave propagating in the metal, of themetal suitably used for the main electrode layer in preferredembodiments of the present invention such as Au, Pt, Ta, Cu, Ni, and Mo,for example, are shown in Table 1.

TABLE 1 Density ρ Transversal bulk wave (kg/m³) c₄₄ [GPa] acousticvelocity v [m/s] Au 19300 29.9 1244.7 Pt 21370 61 1689.5 Ta 16600 81.822220.1 Cu 8930 51.4 2399.1 Ni 8850 92.9 3239.9 Mo 10280 109 3256.2

As illustrated in FIG. 8, it can be seen that, as the acoustic velocityv of the transversal bulk wave propagating in the metal that is the maincomponent of the main electrode layer decreases, the maximum value ofthe duty ratio in the central region at which the acoustic velocityratio V2/V1 is able to be set to about 0.98 or less increases. That is,when metal for which the acoustic velocity of the transversal bulk waveis a certain value or less is not used as the main electrode layer, nomatter how the duty ratio in the first low acoustic velocity region andthe second low acoustic velocity region is set, the value of theacoustic velocity ratio V2/V1 cannot be set to about 0.98 or less exceptfor a case where the duty ratio in the central region is set to a smallvalue equal to or smaller than the above maximum value.

A value of the upper limit of the duty ratio in the central regiondepends on the material of the main electrode layer. Then, the inventorsof preferred embodiments of the present invention have discovered fromthis result that the above upper limit value of the duty ratio in thecentral region is highly correlated with the acoustic velocity v of thetransversal bulk wave propagating in the metal, among physical propertyvalues of the material of the main electrode layer. Thus, in order toachieve reduction or prevention of the spurious components that is theadvantageous effect of the present invention, metals used for the mainelectrode layer are limited. That is, it is necessary to use a metal forwhich the transversal wave acoustic velocity is low as the mainelectrode layer.

Here, when the maximum value of the duty ratio in the central region atwhich the acoustic velocity ratio V2/V1 becomes about 0.98 is defined asD_(M) in FIG. 8, a relational expression between D_(M) and the acousticvelocity v (m/s) of the transversal bulk wave propagating in the metalthat is the main component of the main electrode layer 6 b of the IDTelectrode may be expressed by the following formula.

D _(M)=−0.0001238v+0.8085

For example, in order to set the acoustic velocity ratio V2/V1 to about0.98 or less while setting the duty ratio in the central region of theIDT electrode to about 0.40, it is necessary to use a metal satisfyingv≤3299 m/s for the main electrode layer of the IDT electrode, accordingto the above formula. Note that, when the duty ratio in the centralregion is less than about 0.40, the acoustic velocity V1 in the centralregion becomes higher than that in the case of the duty ratio about0.40, so that the acoustic velocity ratio V2/V1 is able to be set toabout 0.98 or less similarly, by using the metal satisfying the aboveconditions for the main electrode layer.

Next, it is described below that one of the conditions under which theacoustic velocity ratio V2/V1 becomes about 0.98 or less is 2) below.

2) The following Formula 1 is satisfied when λ is a wave length definedby an electrode finger pitch of an IDT electrode, and T is a filmthickness of a main electrode layer normalized by the wave length λ.

T≥0.00018e ^(0.002v)+0.014  Formula 1

Here, since the duty ratio in the central region is set to about 0.40,in order to make the acoustic velocity ratio V2/V1 the minimum value, itis necessary to set V1 to a value when the duty ratio is about 0.40, andto make V2 the minimum value as far as possible during themanufacturing. Since V2<V1 is required, it is necessary to make the dutyratio in the first low acoustic velocity region and the second lowacoustic velocity region larger than the duty ratio in the centralregion, and to make V2 the minimum value within a range of the dutyratio of about 0.40 or more and about 0.80 or less, for example.

Note that, the value when the duty ratio is about 0.40, and the minimumvalue in the range of the duty ratio of about 0.4 or more and about 0.80or less depend on the value of the film thickness of the main electrodelayer made of the metal satisfying the above-described condition 1).Thus, in the following, conditions under which the acoustic velocityratio V2/V1 becomes about 0.98 or less for the material used for themain electrode layer and the film thickness of the main electrode layerwill be discussed in detail.

The duty ratio in the central region is set to about 0.40, and the dutyratio in the first low acoustic velocity region and the second lowacoustic velocity region is set to a duty ratio at which the acousticvelocity becomes the lowest within the range of about 0.40 or more andabout 0.80 or less. In this case, when the main electrode layer is madeof Pt, as illustrated in FIG. 7, the film thickness of the mainelectrode layer at which the acoustic velocity ratio of the first lowacoustic velocity region and the second low acoustic velocity region tothe central region becomes about 0.98 is about 0.019λ.

Similarly, a relationship similar to that illustrated in FIG. 7 wasdetermined also for a case where the metal satisfying the above 1), suchas Au, Cu, Mo, Ta, or Ni, for example, was used for the main electrodelayer. When the duty ratio in the central region was set to about 0.40,and the duty ratio in the first low acoustic velocity region and thesecond low acoustic velocity region was set to the duty ratio at whichthe acoustic velocity becomes the lowest within the range of about 0.40or more and about 0.80 or less, the film thickness T of the mainelectrode layer was determined at which the acoustic velocity ratioV2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region becomes about 0.98. Arelationship between this film thickness T and the acoustic velocity vof the transversal bulk wave propagating in the metal that is the maincomponent of the main electrode layer was determined.

FIG. 9 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer, and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.40 and the acoustic velocity ratio V2/V1 becomes about0.98.

Plots indicated in FIG. 9 are approximately located on a curve that isrepresented by the following formula.

T=0.00018e ^(0.002v)+0.014

By making the film thickness of the main electrode layer larger than thefilm thickness T represented by this formula, the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region is able to be set toabout 0.98 or less. Note that, when the duty ratio in the central regionis smaller than about 0.40, the acoustic velocity V1 in the centralregion becomes higher than that in the case of the duty ratio of about0.40, so that by making the film thickness of the main electrode layerlarger than the film thickness T expressed by the above formula,similarly the acoustic velocity ratio V2/V1 is able to be set to about0.98 or less. Thus, by using the metal satisfying the above 1) for themain electrode layer to satisfy a condition of the following Formula 1,the acoustic velocity ratio V2/V1 is able to be set to about 0.98 orless.

T≥0.00018e ^(0.002v)+0.014  Formula 1

The conditions 1) and 2) are conditions under which the spuriouscomponents are able to be effectively reduced or prevented when the dutyratio in the central region is about 0.4. When the duty ratio in thecentral region is smaller than about 0.4, the acoustic velocity V1 inthe central region becomes higher than that in the case of the dutyratio of about 0.4, so that by using the metal satisfying the aboveconditions 1) and 2) for the main electrode layer, the acoustic velocityratio V2/V1 is able to be set to about 0.98 or less, and the spuriouscomponents are able to be effectively reduced or prevented. That is, byusing the metal satisfying the conditions 1) and 2) for the mainelectrode layer, as the duty ratio in the central region, a range ofabout 0.3 or more and about 0.4 or less, for example, is able to beselected.

In addition, when the film thickness of the main electrode layer isexcessively large, it is difficult to form the IDT electrode, and when adielectric film is provided on the IDT electrode, there is a problemthat a crack is likely to occur in the dielectric layer. Thus, it ispreferable that the film thickness T of the main electrode layer satisfyT≤0.20λ, for example.

As described above, as the duty ratio in the central region is reduced,the acoustic velocity V1 in the central region is able to be increased,and the acoustic velocity ratio V2/V1 is able to be reduced. Thus, thespurious components caused by high-order transverse modes are able to beeffectively reduced or prevented. However, when the duty ratio is set tobe excessively small, electric resistance of the first electrode fingersand the second electrode fingers becomes excessively large, so that aproblem arises in that an insertion loss of the acoustic wave devicebecomes large. Thus, it is preferable to make the duty ratio in thecentral region as large as possible. This makes it possible to reducethe insertion loss of the acoustic wave device. From the above, there isa trade-off relationship between the reduction or prevention of thespurious components and the reduction of the insertion loss. In thepresent preferred embodiment, it is possible to improve the trade-offtherebetween and to make them compatible with each other.

For example, when the duty ratio in the central region of the IDTelectrode is increased to about 0.45, and when the acoustic velocityratio V2/V1 becomes about 0.98 or less, the spurious components are ableto be effectively reduced or prevented, and the electric resistance ofthe IDT electrode is able to be reduced. The inventors of preferredembodiments of the present invention have discovered that the conditionsunder which, when the duty ratio in the central region of the IDTelectrode is set to about 0.45, and the acoustic velocity ratio V2/V1becomes about 0.98 or less are 3) and 4) described later.

3) v≤2895 m/s, where v (m/s) is the acoustic velocity of the transversalbulk wave propagating in the metal that is the main component of themain electrode layer.

4) The following Formula 2 is satisfied when λ is the wave lengthdefined by the electrode finger pitch of the IDT electrode, and T is thefilm thickness of the main electrode layer normalized by the wave lengthλ.

T≥0.000029e ^(0.0032v)+0.02  Formula 2

The following will describe that the conditions under which, when theduty ratio in the central region of the IDT electrode is set to about0.45, and the acoustic velocity ratio V2/V1 becomes about 0.98 or lessare 3) and 4) described above.

First, as illustrated in FIG. 8, when the duty ratio in the centralregion is set to about 0.45, the condition for the acoustic velocity vof the transversal bulk wave of a metal material under which theacoustic velocity ratio is able to be set to about 0.98 or less isv≤2895 m/s. Note that, when the duty ratio in the central region issmaller than about 0.45, the acoustic velocity V1 in the central regionbecomes higher than that in the case of the duty ratio of about 0.45, sothat the acoustic velocity ratio V2/V1 is able to be set to about 0.98or less similarly, by using the metal satisfying the above conditionsfor the main electrode layer.

Then, in the metal satisfying the above condition 3), when the dutyratio is about 0.45, the acoustic velocity becomes the maximum value,and thus, in order to make the acoustic velocity ratio V2/V1 the minimumvalue, V1 is set to a value when the duty ratio is about 0.45, and V2 isset to the minimum value within a range of the duty ratio of about 0.45or more and about 0.8 or less, for example.

Note that, the value when the duty ratio is about 0.45, and the minimumvalue in the range of the duty ratio of about 0.45 or more and about 0.8or less depend on the value of the film thickness of the main electrodelayer made of the metal satisfying the condition 3). Thus, in thefollowing, conditions under which the acoustic velocity ratio V2/V1becomes about 0.98 or less for the material used for the main electrodelayer and the film thickness of the main electrode layer will bediscussed in detail.

The duty ratio in the central region is set to about 0.45, and the dutyratio in the low acoustic velocity region is set to a duty ratio atwhich the acoustic velocity becomes the lowest within a range of about0.45 or more and about 0.80 or less. In this case, when the mainelectrode layer is made of Pt, as illustrated in FIG. 7, the filmthickness of the main electrode layer at which the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region becomes about 0.98 isabout 0.027λ.

Similarly, a relationship similar to that illustrated in FIG. 7 wasdetermined also for the case where the metal satisfying the above 3),such as Au, Cu, or Ta, was used for the main electrode layer. When theduty ratio in the central region was set to the duty ratio about 0.45,and the duty ratio in the first low acoustic velocity region and thesecond low acoustic velocity region was set to a duty ratio at which theacoustic velocity becomes the lowest within the range of about 0.45 ormore and about 0.80 or less, the film thickness T of the main electrodelayer was determined at which the acoustic velocity ratio V2/V1 of thefirst low acoustic velocity region and the second low acoustic velocityregion, and the central region becomes about 0.98. A relationshipbetween this film thickness T and the acoustic velocity v of thetransversal bulk wave propagating in the metal that is the maincomponent of the main electrode layer was determined.

FIG. 10 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.45 and the acoustic velocity ratio V2/V1 becomes about0.98.

Plots indicated in FIG. 10 are approximately located on a curve that isrepresented by the following formula.

T=0.000029e ^(0.0032v)+0.02

By making the film thickness of the main electrode layer larger than thefilm thickness T represented by this formula, the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region is able to be set toabout 0.98 or less. Note that, when the duty ratio in the central regionis smaller than about 0.45, the acoustic velocity V1 in the centralregion becomes higher than that in the case of the duty ratio about0.45, so that by making the film thickness of the main electrode layerlarger than the film thickness T expressed by the above formula,similarly the acoustic velocity ratio V2/V1 is able to be set to about0.98 or less.

Thus, by using the metal satisfying the above 3) as the main electrodelayer to satisfy a condition of the following Formula 2, the acousticvelocity ratio V2/V1 is able to be set to about 0.98 or less.

T≥0.000029e ^(0.0032v)+0.02  Formula 2

The conditions 3) and 4) are conditions under which the spuriouscomponents are able to be effectively reduced or prevented when the dutyratio in the central region is about 0.45. When the duty ratio in thecentral region is smaller than about 0.45, the acoustic velocity V1 inthe central region becomes higher than that in the case of the dutyratio about 0.45, so that by using the metal satisfying the aboveconditions 3) and 4) for the main electrode layer, the acoustic velocityratio V2/V1 is able to be set to about 0.98 or less, and the spuriouscomponents are able to be effectively reduced or prevented. That is, byusing the metal satisfying conditions 3) and 4) for the main electrodelayer, as the duty ratio in the central region, a range of about 0.3 ormore and about 0.45 or less, for example, is able to be selected. Thatis, since it becomes possible to select a large duty ratio, it ispossible to reduce the resistance of the electrode fingers and to reducethe insertion loss of the acoustic wave device.

When the duty ratio in the central region of the IDT electrode isincreased to about 0.50, the electric resistance of the IDT electrode isable to be further reduced or prevented, and the insertion loss is ableto be further reduced. The inventors of preferred embodiments of thepresent invention have discovered that the conditions under which, whilethe duty ratio in the central region of the IDT electrode is set toabout 0.50, and the acoustic velocity ratio V2/V1 becomes about 0.98 orless are 5) and 6) below.

5) v≤2491 m/s, where v (m/s) is the acoustic velocity of the transversalbulk wave propagating in the metal that is the main component of themain electrode layer.

6) The following Formula 3 is satisfied when λ is the wave lengthdefined by the electrode finger pitch of the IDT electrode, and T is thefilm thickness of the main electrode layer normalized by the wave lengthλ.

T≥0.000038e ^(0.0035v)+0.025  Formula 3

First, as illustrated in FIG. 8, when the duty ratio in the centralregion is set to about 0.50, the condition for the acoustic velocity vof the transversal bulk wave of a metal material under which theacoustic velocity ratio is able to be set to about 0.98 or less isv≤2491 m/s. Note that, when the duty ratio in the central region issmaller than about 0.50, the acoustic velocity V1 in the central regionbecomes higher than that in the case of the duty ratio of about 0.50, sothat the acoustic velocity ratio V2/V1 is able to be set to about 0.98or less similarly, by using the metal satisfying the above conditionsfor the main electrode layer.

Then, in the metal satisfying the above condition 5), when the dutyratio is about 0.50, the acoustic velocity becomes the maximum value,and thus, in order to make the acoustic velocity ratio V2/V1 the minimumvalue, V1 is set to a value when the duty ratio is about 0.50, and V2 isset to the minimum value within a range of the duty ratio of about 0.5or more and about 0.8 or less, for example.

Note that, the value when the duty ratio is about 0.50, and the minimumvalue in the range of the duty ratio of about 0.50 or more and about0.80 or less depend on the value of the film thickness of the mainelectrode layer made of the metal satisfying the condition 5). Thus, inthe following, conditions under which the acoustic velocity ratio V2/V1becomes about 0.98 or less for the material used for the main electrodelayer and the film thickness of the main electrode layer will bediscussed in detail.

The duty ratio in the central region is set to about 0.50, and the dutyratio in the low acoustic velocity region is set to a duty ratio atwhich the acoustic velocity becomes the lowest within the range of theduty ratio of about 0.50 or more and about 0.80 or less. In this case,when the main electrode layer is made of Pt, as illustrated in FIG. 7,the film thickness of the main electrode layer at which the acousticvelocity ratio V2/V1 of the first low acoustic velocity region and thesecond low acoustic velocity region, and the central region becomesabout 0.98 is about 0.04λ.

Similarly, a relationship similar to that illustrated in FIG. 7 wasdetermined also for the case where the metal satisfying the above 5),such as Au or Ta, for example, was used for the main electrode layer.When the duty ratio in the central region was set to about 0.50, and theduty ratio in the first low acoustic velocity region and the second lowacoustic velocity region was set to the duty ratio at which the acousticvelocity becomes the lowest within the range of about 0.50 or more andabout 0.80 or less, the film thickness T of the main electrode layer wasdetermined at which the acoustic velocity ratio V2/V1 of the first lowacoustic velocity region and the second low acoustic velocity region,and the central region becomes about 0.98. A relationship between thisfilm thickness T and the acoustic velocity v of the transversal bulkwave propagating in the metal that is the main component of the mainelectrode layer was determined.

FIG. 11 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer, and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.50 and the acoustic velocity ratio V2/V1 becomes about0.98.

Plots indicated in FIG. 11 are approximately located on a curve that isrepresented by the following formula.

T=0.000038e ^(0.0035v)+0.025

By making the film thickness of the main electrode layer larger than thefilm thickness T represented by this formula, the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region is able to be set toabout 0.98 or less. Note that, when the duty ratio in the central regionis smaller than about 0.50, the acoustic velocity V1 in the centralregion becomes higher than that in the case of the duty ratio about0.50, so that by making the film thickness of the main electrode layerlarger than the film thickness T expressed by the above formula,similarly the acoustic velocity ratio V2/V1 is able to be set to about0.98 or less.

Thus, by using the metal satisfying the above 5) as the main electrodelayer to satisfy a condition of the following Formula 3, the acousticvelocity ratio V2/V1 is able to be set to about 0.98 or less.

T≥0.000038e ^(0.0035v)+0.025  Formula 3

The conditions 5) and 6) are conditions under which the spuriouscomponents are able to be effectively reduced or prevented when the dutyratio in the central region is about 0.5. When the duty ratio in thecentral region is smaller than about 0.5, the acoustic velocity V1 inthe central region becomes higher than that in the case of the dutyratio about 0.5, so that by using the metal satisfying the aboveconditions 5) and 6) for the main electrode layer, the acoustic velocityratio V2/V1 is able to be set to about 0.98 or less, and the spuriouscomponents are able to be effectively reduced or prevented. That is, byusing the metal satisfying conditions 5) and 6) for the main electrodelayer, as the duty ratio in the central region, a range of about 0.3 ormore and about 0.5 or less, for example, is able to be selected. Thatis, since it becomes possible to select a further larger duty ratio, itis possible to further reduce the resistance of the electrode fingersand to further reduce the insertion loss of the acoustic wave device.

Here, it is possible to reduce the electric resistance of the IDTelectrode as the duty ratio in the central region of the IDT electrodeis increased, and to reduce the insertion loss. For example, when theduty ratio in the central region of the IDT electrode is increased toabout 0.525, and when the acoustic velocity ratio V2/V1 becomes about0.98 or less, the spurious components are able be effectively reduced orprevented, and the electric resistance of the IDT electrode is able tobe further reduced. The inventors of preferred embodiments of thepresent invention have discovered that the conditions under which, whenthe duty ratio in the central region of the IDT electrode is set toabout 0.525, and the acoustic velocity ratio V2/V1 becomes about 0.98 orless are 7) and 8) described later.

7) v≤2289 m/s, where v (m/s) is the acoustic velocity of the transversalbulk wave propagating in the metal that is the main component of themain electrode layer.

8) The following Formula 4 is satisfied when λ is the wave lengthdefined by the electrode finger pitch of the IDT electrode, and T is thefilm thickness of the main electrode layer normalized by the wave lengthλ.

T≥0.000020e ^(0.0042v)+0.030  Formula 4

The following will describe that the conditions under which, while theduty ratio in the central region of the IDT electrode is set to about0.525, and the acoustic velocity ratio V2/V1 becomes about 0.98 or lessare 7) and 8) described above.

First, as illustrated in FIG. 8, when the duty ratio in the centralregion is set to about 0.525, the condition for the acoustic velocity vof the transversal bulk wave of a metal material under which theacoustic velocity ratio is able to be set to about 0.98 or less isv≤2289 m/s. Note that, when the duty ratio in the central region issmaller than about 0.525, the acoustic velocity V1 in the central regionbecomes higher than that in the case of the duty ratio about 0.525, sothat the acoustic velocity ratio V2/V1 is able to be set to about 0.98or less similarly, by using the metal satisfying the above conditionsfor the main electrode layer.

Then, in the metal satisfying the above condition 7), when the dutyratio is about 0.525, the acoustic velocity becomes the maximum value,and thus, in order to make the acoustic velocity ratio V2/V1 the minimumvalue, V1 is set to a value when the duty ratio is about 0.525, and V2is set to the minimum value within a range of the duty ratio of about0.525 or more and about 0.8 or less, for example.

Note that, the value when the duty ratio is about 0.525, and the minimumvalue in the range of the duty ratio of about 0.525 or more and about0.8 or less depend on the value of the film thickness of the mainelectrode layer made of the metal satisfying the condition 7). Thus, inthe following, conditions under which the acoustic velocity ratio V2/V1becomes about 0.98 or less for the material used for the main electrodelayer and the film thickness of the main electrode layer will bediscussed in detail.

The duty ratio in the central region is set to about 0.525, and the dutyratio in the low acoustic velocity region is set to a duty ratio atwhich the acoustic velocity becomes the lowest within the range of about0.525 or more and about 0.80 or less. In this case, when the mainelectrode layer is made of Pt, as illustrated in FIG. 7, the filmthickness of the main electrode layer at which the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region becomes about 0.98 isabout 0.053λ.

Similarly, a relationship similar to that illustrated in FIG. 7 wasdetermined also for the case where the metal satisfying the above 7),such as Au or Ta, for example, was used for the main electrode layer.When the duty ratio in the central region was set to the duty ratioabout 0.525, and the duty ratio in the first low acoustic velocityregion and the second low acoustic velocity region was set to a dutyratio at which the acoustic velocity becomes the lowest within the rangeof about 0.525 or more and about 0.80 or less, the film thickness T ofthe main electrode layer was determined at which the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region becomes about 0.98. Arelationship between this film thickness T and the acoustic velocity vof the transversal bulk wave propagating in the metal that is the maincomponent of the main electrode layer was determined.

FIG. 12 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer, and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.525 and the acoustic velocity ratio V2/V1 becomesabout 0.98.

Plots indicated in FIG. 12 are approximately located on a curve that isrepresented by the following formula.

T=0.000020e ^(0.0042v)+0.030

By making the film thickness of the main electrode layer larger than thefilm thickness T represented by this formula, the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region is able to be set toabout 0.98 or less. Note that, when the duty ratio in the central regionis smaller than about 0.525, the acoustic velocity V1 in the centralregion becomes higher than that in the case of the duty ratio about0.525, so that by making the film thickness of the main electrode layerlarger than the film thickness T expressed by the above formula,similarly the acoustic velocity ratio V2/V1 is able to be set to about0.98 or less.

Thus, by using the metal satisfying the condition 7) above as the mainelectrode layer to satisfy a condition of the following Formula 4, theacoustic velocity ratio V2/V1 is able to be set to about 0.98 or less.

T≥0.000020e ^(0.0042v)+0.030  Formula 4

The conditions 7) and 8) are conditions under which the spuriouscomponents are able to be effectively reduced or prevented when the dutyratio in the central region is about 0.525. When the duty ratio in thecentral region is smaller than about 0.525, the acoustic velocity V1 inthe central region becomes higher than that in the case of the dutyratio about 0.525, so that by using the metal satisfying the aboveconditions 7) and 8) for the main electrode layer, the acoustic velocityratio V2/V1 is able to be set to 0.98 or less, and the spuriouscomponents are able to be effectively reduced or prevented. That is, byusing the metal satisfying the conditions 7) and 8) for the mainelectrode layer, as the duty ratio in the central region, a range ofabout 0.3 or more and about 0.525 or less is able to be selected. Thatis, since it becomes possible to select a further larger duty ratio, itis possible to further reduce the resistance of the electrode fingersand to further reduce the insertion loss of the acoustic wave device.

Here, it is possible to reduce the electric resistance of the IDTelectrode as the duty ratio in the central region of the IDT electrodeis increased, and to reduce the insertion loss. For example, when theduty ratio in the central region of the IDT electrode is increased toabout 0.55, and when the acoustic velocity ratio V2/V1 becomes about0.98 or less, the spurious components are able to be effectively reducedor prevented, and the electric resistance of the IDT electrode is ableto be further reduced. The inventors of preferred embodiments of thepresent invention have discovered that the conditions under which, whenthe duty ratio in the central region of the IDT electrode is set toabout 0.55, and the acoustic velocity ratio V2/V1 becomes about 0.98 orless are 9) and 10) described later.

9) v≤2087 m/s, where v (m/s) is the acoustic velocity of the transversalbulk wave propagating in the metal that is the main component of themain electrode layer.

10) The following Formula 5 is satisfied when λ is the wave lengthdefined by the electrode finger pitch of the IDT electrode, and T is thefilm thickness of the main electrode layer normalized by the wave lengthλ.

T≥0.000017e ^(0.0048v)+0.033  Formula 5

The following will describe that the conditions under which, while theduty ratio in the central region of the IDT electrode is set to about0.55, and the acoustic velocity ratio V2/V1 becomes about 0.98 or lessare 9) and 10).

First, as illustrated in FIG. 8, when the duty ratio in the centralregion is set to about 0.55, the condition for the acoustic velocity vof the transversal bulk wave of a metal material under which theacoustic velocity ratio can be set to about 0.98 or less is v≤2087 m/s.Note that, when the duty ratio in the central region is smaller thanabout 0.55, the acoustic velocity V1 in the central region becomeshigher than that in the case of the duty ratio of about 0.55, so thatthe acoustic velocity ratio V2/V1 is able to be set to about 0.98 orless similarly, by using the metal satisfying the above conditions forthe main electrode layer.

Then, in the metal satisfying the above condition 9), when the dutyratio is about 0.55, the acoustic velocity becomes the maximum value,and thus, in order to make the acoustic velocity ratio V2/V1 the minimumvalue, V1 is set to a value when the duty ratio is about 0.55, and V2 isset to the minimum value within a range of the duty ratio of about 0.55or more and about 0.8 or less, for example.

Note that, the value when the duty ratio is about 0.55, and the minimumvalue in the range of the duty ratio of about 0.55 or more and about 0.8or less depend on the value of the film thickness of the main electrodelayer made of the metal satisfying the condition 9). Thus, in thefollowing, conditions under which the acoustic velocity ratio V2/V1becomes about 0.98 or less for the material used for the main electrodelayer and the film thickness of the main electrode layer will bediscussed in detail.

The duty ratio in the central region is set to about 0.55, and the dutyratio in the low acoustic velocity region is set to a duty ratio atwhich the acoustic velocity becomes the lowest within a range of about0.55 or more and about 0.80 or less. In this case, when the mainelectrode layer is made of Pt, as illustrated in FIG. 7, the filmthickness of the main electrode layer at which the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region becomes about 0.98 isabout 0.090λ.

Similarly, a relationship similar to that illustrated in FIG. 7 wasdetermined also for the case where the metal satisfying the above 9),such as Au, for example, was used for the main electrode layer. When theduty ratio in the central region was set to the duty ratio about 0.55,and the duty ratio in the first low acoustic velocity region and thesecond low acoustic velocity region was set to a duty ratio at which theacoustic velocity becomes the lowest within the range of about 0.55 ormore and about 0.80 or less, the film thickness T of the main electrodelayer was determined at which the acoustic velocity ratio V2/V1 of thefirst low acoustic velocity region and the second low acoustic velocityregion, and the central region becomes about 0.98. A relationshipbetween this film thickness T and the acoustic velocity v of thetransversal bulk wave propagating in the metal that is the maincomponent of the main electrode layer was determined.

FIG. 13 is a diagram illustrating a relationship between the acousticvelocity v of the transversal bulk wave propagating in the metal that isthe main component of the main electrode layer, and the film thickness Tof the main electrode layer at which the duty ratio in the centralregion is about 0.55 and the acoustic velocity ratio V2/V1 becomes about0.98.

Plots indicated in FIG. 13 are approximately located on a curve that isrepresented by the following formula.

T=0.000017e ^(0.0048v)+0.033

By making the film thickness of the main electrode layer larger than thefilm thickness T represented by this formula, the acoustic velocityratio V2/V1 of the first low acoustic velocity region and the second lowacoustic velocity region, and the central region is able to be set toabout 0.98 or less. Note that, when the duty ratio in the central regionis smaller than about 0.55, the acoustic velocity V1 in the centralregion becomes higher than that in the case of the duty ratio about0.55, so that by making the film thickness of the main electrode layerlarger than the film thickness T expressed by the above formula, theacoustic velocity ratio V2/V1 is able to be set to about 0.98 or less.

Thus, by using the metal satisfying the above 9) as the main electrodelayer to satisfy a condition of the following Formula 5, the acousticvelocity ratio V2/V1 is able to be set to about 0.98 or less.

T≥0.000017e ^(0.0048v)+0.033  Formula 5

The conditions 9) and 10) are conditions under which the spuriouscomponents are able to be effectively reduced or prevented when the dutyratio in the central region is about 0.55. When the duty ratio in thecentral region is smaller than about 0.55, the acoustic velocity V1 inthe central region becomes higher than that in the case of the dutyratio about 0.55, so that by using the metal satisfying the aboveconditions 9) and 10) for the main electrode layer, the acousticvelocity ratio V2/V1 is able to be set to about 0.98 or less, and thespurious components are able to be effectively reduced or prevented.That is, by using the metal satisfying conditions 9) and 10) for themain electrode layer, as the duty ratio in the central region, a rangeof about 0.3 or more and about 0.55 or less is able to be selected. Thatis, since it becomes possible to select a further larger duty ratio, itis possible to further reduce the resistance of the electrode fingersand to further reduce the insertion loss of the acoustic wave device.

As described above, in the first preferred embodiment, the IDT electrodeis a multilayer body including the main electrode layer and theconductive auxiliary electrode layer. Note that, it is sufficient thatthe IDT electrode includes the main electrode layer, and layeredstructure is not limited to the above. Examples of layered structure ofthe IDT electrodes are described below by first to third modificationexamples of the first preferred embodiment.

In the first modification example of the first preferred embodimentillustrated in FIG. 14, an adhesion layer 6 a is provided on thepiezoelectric substrate 2. The main electrode layer 6 b is laminated onthe adhesion layer 6 a. The adhesion layer 6 a is preferably made of,for example, Ti or NiCr. Since the adhesion layer 6 a is provided,adhesion of an IDT electrode 13 to the piezoelectric substrate 2 isimproved.

In an IDT electrode 23 in the second modification example of the firstpreferred embodiment illustrated in FIG. 15, the adhesion layer 6 a, themain electrode layer 6 b, and an adhesion layer 6 e are laminated inthis order from a side of the piezoelectric substrate 2.

In an IDT electrode 33 in the third modification example of the firstpreferred embodiment illustrated in FIG. 16, the adhesion layer 6 a, themain electrode layer 6 b, a diffusion prevention layer 6 c, theconductive auxiliary electrode layer 6 d, and the adhesion layer 6 e arelaminated in this order from the side of the piezoelectric substrate 2.The diffusion prevention layer 6 c is preferably made of, for example,Ti. By providing the diffusion prevention layer 6 c, interdiffusionbetween the main electrode layer 6 b and the conductive auxiliaryelectrode layer 6 d is less likely to occur. Thus, deterioration of theIDT electrode 33 is less likely to occur.

Here, in the first preferred embodiment illustrated in FIG. 1, the firsthigh acoustic velocity region and the second high acoustic velocityregion are provided between the first busbar 3 a 1 and the first edgeregion A2 a and between the second busbar 3 b 1 and the second edgeregion A2 b, respectively. Note that, the first high acoustic velocityregion and the second high acoustic velocity region may be provided inthe first busbar 3 a 1 and in the second busbar 3 b 1, respectively.This is described in a fourth modification example of the firstpreferred embodiment described below.

FIG. 17 is an enlarged plan view illustrating a vicinity of a firstbusbar in the fourth modification example of the first preferredembodiment.

In the present modification example, a plurality of cavities 55 isprovided in a first busbar 53 a 1, and the first busbar 53 a 1 isdivided into an inner busbar portion 53A, a central busbar portion 53B,and an outer busbar portion 53C. The inner busbar portion 53A definesand functions as a first low acoustic velocity region together with afirst edge region, and the central busbar portion 53B defines andfunctions as a first high acoustic velocity region. Here, when acousticvelocity in the inner busbar portion 53A is defined as V4, acousticvelocity in the central busbar portion 53B is defined as V5, andacoustic velocity in the outer busbar portion 53C is defined as V6, thenV5 is the highest acoustic velocity in the entire region. Since each ofthe regions where the acoustic velocity becomes V2 to V4 defines andfunctions as a low acoustic velocity region, and the region where theacoustic velocity becomes V5 defines and functions as a high acousticvelocity region, an acoustic velocity relationship among the regions isrepresented by V5>V1>(an average of V2 to V4), and a piston mode isestablished.

The above-described relationship among the respective acousticvelocities V1 to V6 is illustrated in FIG. 17. It is illustrated that inFIG. 17, the acoustic velocity on the outer side is higher. Note that, aside of the second busbar is also configured in the same orsubstantially the same manner as a side of the first busbar 53 a 1.

In the first preferred embodiment, the piezoelectric body is preferablythe piezoelectric substrate 2, for example, but the piezoelectric bodymay be a piezoelectric thin film 42 as in a fifth modification exampleof the first preferred embodiment illustrated in FIG. 18. For example, alow acoustic velocity film 43 may be provided on a surface opposite to asurface on which the IDT electrode 3 of the piezoelectric thin film 42is provided. A high acoustic velocity member 44 may be provided on asurface of the low acoustic velocity film 43 opposite to a side of thepiezoelectric thin film 42.

Here, the low acoustic velocity film 43 is a film in which the acousticvelocity of a bulk wave propagating in the piezoelectric thin film 42 islower than that of an acoustic wave propagating in the piezoelectricthin film 42. The low acoustic velocity film 43 is preferably made of,for example, a material containing, as a main component, a compoundobtained by adding fluorine, carbon or boron to glass, silicon oxide,silicon oxynitride, tantalum oxide or silicon oxide, or the like. Notethat, as the material of the low acoustic velocity film 43, a materialof which acoustic velocity is relatively low is sufficient.

The high acoustic velocity member 44 is a member in which the acousticvelocity of the bulk wave propagating in the piezoelectric thin film 42is higher than that of the acoustic wave propagating in thepiezoelectric thin film 42. The high acoustic velocity member 44 ispreferably made of, for example, a material containing aluminum nitride,aluminum oxide, silicon carbide, silicon oxynitride, silicon, a DLCfilm, or diamond as a main component. Note that, as the material of thehigh acoustic velocity member 44, a material for which acoustic velocityis relatively high is sufficient.

The high acoustic velocity member 44 may be a high acoustic velocityfilm or a high acoustic velocity substrate. In this manner, when the lowacoustic velocity film 43 and the high acoustic velocity member 44 areprovided, energy of an acoustic wave is effectively confined.

In the past, the spurious components other than the spurious componentscaused by high-order transverse modes reduced or prevented by a pistonmode occurred in a vicinity of a pass band in some cases. For example,for an acoustic wave device using a Rayleigh wave, an SH wave becomesthe spurious component, and for an acoustic wave device using the SHwave such as a Love wave, the Rayleigh wave becomes the spuriouscomponent.

In an acoustic wave device according to a second preferred embodiment ofthe present invention described below, when the Rayleigh wave is used,in addition to the spurious components caused by high-order transversemodes, the spurious components caused by the SH wave are able to bereduced or prevented.

In the acoustic wave device of the second preferred embodiment, apiezoelectric substrate is preferably made of, for example, LiNbO₃, andEuler angles (φ, θ, ψ) of the piezoelectric substrate are defined asdescribed below. Further, a first dielectric film preferably made ofsilicon oxide is provided. Other than the above points, the acousticwave device according to the second preferred embodiment has the same orsubstantially the same structure as that of the acoustic wave device 1according to the first preferred embodiment illustrated in FIG. 1. Notethat, in the second preferred embodiment, the Rayleigh wave is used.

More specifically, in the second preferred embodiment, the Euler angles(φ, θ, ψ) of the piezoelectric substrate are Euler angles (0°±5°, θ,0°±10′). θ in the Euler angles (φ, θ, ψ) satisfies θ≥27°.

Here, a ratio of density of a material of a main electrode layer todensity ρ_(Pt) of Pt is defined as r=p/ρ_(Pt). At this time, the Eulerangles (φ, θ, ψ) of the piezoelectric substrate 2 are preferably (0°±5°,{−0.054/(T×r−0.044)+31.33}°±1.5°, 0°±10°). This makes it possible tofurther reduce or prevent the spurious components caused by the SH wave,that matters in the case of using the Rayleigh wave. In addition, in thesame or similar manner as in the first preferred embodiment, it ispossible to reduce or prevent the spurious components caused byhigh-order transverse modes. Note that, details of the above relationalexpression between θ in the Euler angles (φ, θ, ψ) and the filmthickness T of the main electrode layer will be described later.

Advantageous effects in the second preferred embodiment will bedescribed in more detail below.

In the second preferred embodiment, Pt, for example, was preferably usedfor the main electrode layer, and impedance frequency characteristicsand a return loss were measured. Here, a frequency was defined as anormalized frequency with a resonant frequency being 1. Note that,conditions are as follows.

Piezoelectric substrate: material LiNbO₃, Euler angles (0°, 30°, 0°)

Main electrode layer: material Pt, film thickness about 0.085λ

Conductive auxiliary electrode layer: material Al, film thickness about0.08λ

First dielectric film: material SiO₂, film thickness about 0.30λ

Second dielectric film: material SiN, film thickness about 0.01λ

Duty ratio in central region: about 0.50

Acoustic wave used: Rayleigh wave

Furthermore, impedance frequency characteristics and a return loss in acomparative example, in which θ in Euler angles (φ, θ, ψ) of apiezoelectric substrate is out of the range of the second preferredembodiment, were measured. Note that, conditions are as follows. In thecomparative example, film structure of an IDT electrode is the same orsubstantially the same as that in the second preferred embodiment, butthe Euler angles of the piezoelectric substrate are set as a conditionin an acoustic wave device using the conventional Rayleigh wave.

Piezoelectric substrate: material LiNbO₃, Euler angles (0°, 38°, 0°)

Main electrode layer: material Pt, film thickness about 0.085λ

Conductive auxiliary electrode layer: material Al, film thickness about0.08λ

First dielectric film: material SiO₂, film thickness about 0.30λ

Second dielectric film: material SiN, film thickness about 0.01λ

Duty ratio in central region: about 0.50

Acoustic wave used: Rayleigh wave

FIG. 19 is a diagram illustrating the impedance frequencycharacteristics of the acoustic wave device according to the secondpreferred embodiment. FIG. 20 is a diagram illustrating the return lossof the acoustic wave device according to the second preferredembodiment. FIG. 21 is a diagram illustrating the impedance frequencycharacteristics of the acoustic wave device of the comparative example.FIG. 22 is a diagram illustrating the return loss of the acoustic wavedevice of the comparative example.

As illustrated in FIG. 21 and FIG. 22, it can be seen that a largespurious component is generated on a lower side of the resonantfrequency in the comparative example. This spurious component is causedby the SH wave.

On the other hand, as illustrated in FIG. 19 and FIG. 20, in the secondpreferred embodiment, it can be seen that the spurious component causedby the SH wave is reduced or prevented. An electromechanical couplingcoefficient of the SH wave varies depending on a value of θ in the Eulerangles (φ, θ, ψ) of the piezoelectric substrate, the film thickness ofthe main electrode layer of the IDT electrode, or the like. In thesecond preferred embodiment, by setting the Euler angles (φ, θ, ψ)within the above range, the electromechanical coupling coefficient ofthe SH wave is able to be set to 0 or approximately 0. Thus, thespurious component caused by the SH wave is able to be effectivelyreduced or prevented. Note that, as illustrated in FIG. 19 and FIG. 20,the spurious components caused by high-order transverse modes are notsubstantially generated.

The above relational expression between θ in the Euler angles (φ, θ, ψ)and the film thickness T of the main electrode layer was determined asfollows. When the value of θ in the Euler angles (φ, θ, ψ) of thepiezoelectric substrate was changed, the film thickness of the mainelectrode layer at which the electromechanical coupling coefficient ofthe SH wave became 0 or approximately 0 was determined. The value of θand the film thickness of the main electrode layer are shown in Table 2below. Note that, Pt was used for the main electrode layer.

TABLE 2 Film thickness (λ) of main electrode layer (Pt) θ (°) 0.045 230.0475 25 0.05 26.5 0.055 28 0.06 29 0.065 29.25 0.07 29.75 0.0775 300.0875 30.25 0.0975 30.5 0.1 30.5

The relationship between θ in the Euler angles (φ, θ, ψ) of thepiezoelectric substrate and the film thickness of the main electrodelayer shown in Table 2 is expressed by the following formula. Note that,the film thickness of the main electrode layer when Pt is used for themain electrode layer is defined as T_(Pt).

θ=−0.054/(T _(Pt)−0.044)+31.33

Note that, in Table 2, under conditions where the film thickness of themain electrode layer made of Pt is relatively small, and θ in the Eulerangles (φ, θ, ψ) is relatively small, the value of θ at which theelectromechanical coupling coefficient of the SH wave becomes 0 orapproximately 0 greatly changes with respect to the change in the filmthickness of the main electrode layer. On the other hand, underconditions where the film thickness of the main electrode layer isrelatively large, and the above θ is relatively large, the change in thevalue of θ at which the electromechanical coupling coefficient of the SHwave becomes 0 or approximately 0 is extremely small with respect to thechange in the film thickness of the main electrode layer. That is, underthe conditions where the value of the above θ is relatively small, avariation in magnitude of the electromechanical coupling coefficient ofthe SH wave is large with respect to a variation in the film thicknessof the main electrode layer due to variation in manufacturing or thelike, and a variation in magnitude of the spurious component caused bythe SH wave also becomes large. Thus, it is preferable that θ in theEuler angles (φ, θ, ψ) be in a range of θ≥27°.

When metal other than Pt is used for the main electrode layer, it hasbeen discovered that it is sufficient that the film thickness T of themain electrode layer is set to a film thickness converted by using aratio r=ρ/ρ_(Pt) of the density ρ of the material of the main electrodelayer to the density ρ_(Pt) of Pt. That is, even when metal other thanPt is used for the main electrode layer, the relationship between θ inthe Euler angles (φ, θ, ψ) and the film thickness T of the mainelectrode layer may be expressed by the following formula, when theelectromechanical coupling coefficient of the SH wave becomes 0.

θ=−0.054/(T×r−0.044)+31.33

Note that, it is also possible to set the electromechanical couplingcoefficient of the SH wave to a value close to 0, even when θ in theEuler angles (φ, θ, ψ) is within a range of{−0.054/(T×r−0.044)+31.33}°±1.5°. Thus, the spurious component caused bythe SH wave is able to be effectively reduced or prevented.

FIG. 23 is a diagram illustrating the range of θ in the Euler angles (φ,θ, ψ) of the piezoelectric substrate according to the second preferredembodiment. In FIG. 23, a solid line indicates a relationship ofθ=−0.054/(T×r−0.044)+31.33. Dashed lines show relationships ofθ={−0.054/(T×r−0.044)+31.33}+1.5 and θ={−0.054/(T×r−0.044)+31.33}−1.5,respectively. Alternate long and short dash lines indicate T×r=0.10λ andθ=27°, respectively.

In the second preferred embodiment, θ in the Euler angles (φ, θ, ψ) is avalue within a range surrounded by a broken line and the alternate longand short dash lines in FIG. 23. As described above, since θ is withinthe range of {−0.054/(T×r−0.044)+31.33}°±1.5°, it is possible toeffectively reduce or prevent the spurious component caused by the SHwave. In the second preferred embodiment, since θ≥27° is satisfied, thespurious component caused by the SH wave is able to be stably reduced orprevented. In addition, since T×r≤0.10λ is satisfied, the IDT electrodeis able to be suitably formed at the time of manufacturing, and a crackis less likely to occur in a dielectric film.

As described above, preferred embodiments of the present inventionclarify that, it is possible to effectively reduce or prevent thespurious components when the acoustic velocity ratio V2/V1 is set toabout 0.98 or less.

In Japanese Unexamined Patent Application Publication No. 2013-518455,there is no description regarding the acoustic velocity ratio V2/V1. Inaddition, in the structure in Japanese Unexamined Patent ApplicationPublication No. 2013-518455, although there is a case where the spuriouscomponents occur depending on the acoustic velocity ratio V2/V1,preferred embodiments of the present invention are able to effectivelyreduce or prevent the spurious components by setting the acousticvelocity ratio V2/V1 to about 0.98 or less.

Further, the above conditions are achieved both in the configurationillustrated in FIG. 1 and the configuration illustrated in FIG. 17.

Further, the above conditions are achieved not only by the method inwhich the first and second low acoustic velocity regions are provided bymaking the duty ratio in the first and second edge regions larger thanthe duty ratio in the central region, but also by the method in whichthe mass addition film made of the dielectric or the metal is layered onthe first and second electrode fingers to provide the low acousticvelocity region.

In the first and second preferred embodiments and the first to fourthmodification examples of the first preferred embodiment, the examples inwhich the acoustic wave device is the one-port acoustic wave resonatorare illustrated. Note that, the present invention may also be suitablyapplied to an acoustic wave device other than the above.

The above-described acoustic wave devices are able to be used as aduplexer or the like of a high-frequency front end circuit. An examplethereof will be described below.

FIG. 24 is a block diagram of a communication device having ahigh-frequency front end circuit according to a preferred embodiment ofthe present invention. In addition, in the figure, respective componentsconnected to a high-frequency front end circuit 230, for example, anantenna element 202 and an RF signal processing circuit (RFIC) 203 arealso illustrated. The high-frequency front end circuit 230 and the RFsignal processing circuit 203 define a communication device 240. Notethat, the communication device 240 may include, for example, a powersupply, a CPU, a display, and the like.

The high-frequency front end circuit 230 includes a switch 225,duplexers 201A, 201B, filters 231, 232, low-noise amplifier circuits214, 224, and power amplifier circuits 234 a, 234 b, 244 a, and 244 b.Note that, the high-frequency front end circuit 230 and thecommunication device 240 illustrated in FIG. 24 are examples of ahigh-frequency front end circuit and a communication device, and are notlimited to this configuration.

The duplexer 201A includes filters 211 and 212. The duplexer 201Bincludes filters 221 and 222. The duplexers 201A and 201B are connectedwith the antenna element 202 with the switch 225 interposedtherebetween. Note that, acoustic wave devices according to preferredembodiments of the present invention may be used for the duplexers 201A,201B, or may be used for the filters 211, 212, 221, 222. The acousticwave devices according to preferred embodiments of the present inventionmay be acoustic wave resonators that define the duplexers 201A, 201B, orthe filters 211, 212, 221, 222.

Further, the acoustic wave devices according to preferred embodiments ofthe present invention may be applied to a multiplexer including three ormore filters such as, for example, a triplexer including three antennaterminals common to each other, and a hexaplexer including six filterantenna terminals common to each other.

That is, the acoustic wave devices according to preferred embodiments ofthe present invention include the acoustic wave resonators, the filters,the duplexers, and the multiplexers including the three or more filters.In addition, the multiplexer is not limited to a configuration includingboth a transmission filter and a reception filter, and may be configuredto include only a transmission filter or a reception filter.

The switch 225 connects the antenna element 202 to a signal pathcorresponding to a predetermined band in accordance with a controlsignal from a control unit (not shown), and is defined by, for example,a Single Pole Double Throw (SPDT) switch. Note that, the number ofsignal paths connected to the antenna element 202 is not limited to one,and may be a plurality of signal paths. That is, the high-frequencyfront end circuit 230 may support carrier aggregation.

The low-noise amplifier circuit 214 is a reception amplifier circuitthat amplifies a high-frequency signal (here, a high-frequency receptionsignal) passing through the antenna element 202, the switch 225, and theduplexer 201A, and outputs to the RF signal processing circuit 203. Thelow-noise amplifier circuit 224 is a reception amplifier circuit thatamplifies a high-frequency signal (here, a high-frequency receptionsignal) passing through the antenna element 202, the switch 225, and theduplexer 201B, and outputs to the RF signal processing circuit 203.

Each of the power amplifier circuits 234 a and 234 b is a transmissionamplifier circuit that amplifies a high-frequency signal (here, ahigh-frequency transmission signal) outputted from the RF signalprocessing circuit 203, and outputs to the antenna elements 202 via theduplexer 201A and the switch 225. Each of the power amplifier circuits244 a and 244 b is a transmission amplifier circuit that amplifies ahigh-frequency signal (here, a high-frequency transmission signal)outputted from the RF signal processing circuit 203, and outputs to theantenna elements 202 via the duplexer 201B and the switch 225.

The RF signal processing circuit 203 applies signal processing to ahigh-frequency reception signal inputted from the antenna element 202via a reception signal path by down-conversion or the like, and outputsa reception signal generated by the signal processing. Further, the RFsignal processing circuit 203 applies signal processing to an inputtedtransmission signal by up-converting or the like, and outputs ahigh-frequency transmission signal generated by the signal processing tothe power amplifier circuits 234 b or 244 b. The RF signal processingcircuit 203 is preferably, for example, an RFIC. Note that, thecommunication device may include a Base Band (BB) IC. In this case, theBBIC applies signal processing to a reception signal processed by theRFIC. Further, the BBIC applies signal processing to a transmissionsignal and outputs to the RFIC. The reception signal processed by theBBIC or a transmission signal before subjected to the signal processingby the BBIC is preferably, for example, an image signal, a sound signal,or the like. Note that, the high-frequency front end circuit 230 mayinclude other circuit elements between the respective componentsdescribed above.

Note that, the high-frequency front end circuit 230 may includeduplexers according to the modification examples of the duplexers 201Aand 201B in place of the duplexers 201A and 201B.

On the other hand, the filters 231 and 232 in the communication device240 are connected between the RF signal processing circuit 203 and theswitch 225 without the low-noise amplifier circuits 214, 224 and thepower amplifier circuits 234 a, 234 b, 244 a, and 244 b interposedtherebetween. The filters 231 and 232 are also connected with theantenna element 202 with the switch 225 interposed therebetween,similarly to the duplexers 201A and 201B.

According to the high-frequency front end circuit 230 and thecommunication device 240 configured as described above, by including theacoustic wave resonator, the filter, the duplexer, the multiplexerincluding three or more number of the filters, and the like, that aredefined by acoustic wave devices according to preferred embodiments ofthe present invention, it is possible to effectively reduce or preventthe spurious components caused by high-order transverse modes.

Although the acoustic wave devices, the high-frequency front endcircuits, and the communication devices according to the preferredembodiments of the present invention have been described with referenceto the preferred embodiments and the modification examples thereof,another preferred embodiment provided by combining arbitrary elements inthe above-described preferred embodiments and modification examplesthereof, modification examples obtained by applying variousmodifications that those skilled in the art think about to theabove-described preferred embodiments within the scope of the presentinvention, and various devices incorporating the high-frequency frontend circuits and the communication devices according to preferredembodiments of the present invention are also included in the presentinvention.

Preferred embodiments of the present invention are widely applicable toa communication apparatus, such as a cellular phone, for example, as amultiplexer, a front end circuit, a communication device, that may beapplied to an acoustic wave resonator, a filter, a duplexer, and amultiband system.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. An acoustic wave device, comprising: apiezoelectric body; and an IDT electrode provided on the piezoelectricbody and including a main electrode layer; wherein the IDT electrodeincludes a first busbar and a second busbar facing each other, aplurality of first electrode fingers first ends of which are connectedwith the first busbar, and a plurality of second electrode fingers firstends of which are connected with the second busbar, the plurality ofsecond electrode fingers being interdigitated with the plurality offirst electrode fingers; the IDT electrode further includes anintersecting region in which the plurality of first electrode fingersand the plurality of second electrode fingers overlap each other in anacoustic wave propagation direction; when a direction in which theplurality of first electrode fingers extend or a direction in which theplurality of second electrode fingers extend is defined as a lengthdirection, the intersecting region includes a central region located ata central portion of the first electrode fingers and the secondelectrode fingers in the length direction, a first low acoustic waveregion which is disposed on an outside of the central region on a sideof the first busbar in the length direction and in which an acousticvelocity is lower than an acoustic velocity in the central region, and asecond low acoustic velocity region which is disposed on an outside ofthe central region on a side of the second busbar in the lengthdirection and in which an acoustic velocity is lower than the acousticvelocity in the central region; a first high acoustic velocity regionwhich is disposed on an outside of the first low acoustic velocityregion on the side of the first busbar in the length direction and inwhich an acoustic velocity is higher than the acoustic velocity in thecentral region, and a second high acoustic velocity region which isdisposed on an outside of the second low acoustic velocity region on theside of the second busbar in the length direction and in which anacoustic velocity is higher than the acoustic velocity in the centralregion are provided; a duty ratio in the first low acoustic velocityregion and the second low acoustic velocity region is greater than aduty ratio in the central region; v≤3299 m/s is satisfied, where v (m/s)represents an acoustic velocity of a transversal bulk wave propagatingin metal that is a main component of the main electrode layer; and whenλ represents a wave length defined by an electrode finger pitch of theIDT electrode and T represents a film thickness of the main electrodelayer normalized by the wave length λ, T≥0.00018e^(0.002)v+0.014 issatisfied.
 2. The acoustic wave device according to claim 1, wherein theIDT electrode includes a plurality of layers including the mainelectrode layer.
 3. The acoustic wave device according to claim 1,wherein the main electrode layer includes any one of Au, Pt, Ta, Cu, Ni,and Mo as a main component.
 4. The acoustic wave device according toclaim 1, wherein in the IDT electrode, v≤2895 m/s is satisfied, andT≥0.000029e^(0.0032v)+0.02 is satisfied.
 5. The acoustic wave deviceaccording to claim 1, wherein in the IDT electrode, v≤2491 m/s issatisfied, and T≥0.000038e^(0.0035v)+0.025 is satisfied.
 6. The acousticwave device according to claim 1, wherein in the IDT electrode, v≤2289m/s is satisfied, and T≥0.000020e_(0.0042v)+0.03 is satisfied.
 7. Theacoustic wave device according to claim 1, wherein in the IDT electrode,v≤2087 m/s is satisfied, and T≥0.000017e^(0.0048v)+0.033 is satisfied.8. The acoustic wave device according to claim 1, wherein thepiezoelectric body is made of LiNbO₃, Euler angles (φ, θ, ψ) of thepiezoelectric body are Euler angles (0°±5°, θ, 0°±10°), θ in the Eulerangles (φ, θ, ψ) of the piezoelectric body satisfies θ≥27°; and theEuler angles (φ, θ, ψ) are (0°±5°, {−0.054/(T×r−0.044)+31.33}°±1.5°,0°±10°), and T×r≤0.10λ is satisfied, where a ratio of density ρ of amaterial of the main electrode layer to density ρ_(Pt) of Pt is definedas r=ρ/ρ_(Pt).
 9. The acoustic wave device according to claim 1, whereinthe first busbar and the second busbar of the IDT electrode includecavities; in the first and second busbars, a portion located closer tothe central region in the length direction than the cavity is an innerbusbar portion, a portion opposite to the inner busbar portion with thecavity interposed is an outer busbar portion; in the first busbar, theinner busbar portion is a low acoustic velocity region, a region inwhich the cavity is provided is the first high acoustic velocity region;and in the second busbar, the inner busbar portion is a low acousticvelocity region, and a region in which the cavity is provided is thesecond high acoustic velocity region.
 10. The acoustic wave deviceaccording to claim 1, further comprising a dielectric film provided onthe piezoelectric body and covering the IDT electrode.
 11. An acousticwave device, comprising: a piezoelectric body; and an IDT electrodeprovided on the piezoelectric body and including a main electrode layer;wherein the IDT electrode includes a first busbar and a second busbarfacing each other, a plurality of first electrode fingers first ends ofwhich are connected with the first busbar, and a plurality of secondelectrode fingers first ends of which are connected with the secondbusbar, the plurality of second electrode fingers are interdigitatedwith the plurality of first electrode fingers; the IDT electrode furtherincludes an intersecting region in which the plurality of firstelectrode fingers and the plurality of second electrode fingers overlapeach other in an acoustic wave propagation direction; when a directionin which the plurality of first electrode fingers extends or a directionin which the plurality of second electrode fingers extends is defined asa length direction, the intersecting region includes, a central regionlocated at a central portion in the first electrode fingers and thesecond electrode fingers in the length direction, a first low acousticwave region which is disposed on an outside of the central region on aside of the first busbar in the length direction and in which anacoustic velocity is lower than an acoustic velocity in the centralregion, and a second low acoustic velocity region which is disposed onan outside of the central region on a side of the second busbar in thelength direction and in which an acoustic velocity is lower than theacoustic velocity in the central region; a first high acoustic velocityregion which is disposed on an outside of the first low acousticvelocity region on a side of the first busbar in the length directionand in which an acoustic velocity is higher than the acoustic velocityin the central region, and a second high acoustic velocity region whichis disposed on an outside of the second low acoustic velocity region ona side of the second busbar in the length direction and in which anacoustic velocity is higher than the acoustic velocity in the centralregion are provided; and V2/V1≤0.98 is satisfied, where V1 representsthe acoustic velocity in the central region and V2 represents theacoustic velocity in the first low acoustic velocity region and thesecond low acoustic velocity region.
 12. The acoustic wave deviceaccording to claim 11, further comprising a dielectric film provided onthe piezoelectric body and covering the IDT electrode.
 13. The acousticwave device according to claim 11, wherein the IDT electrode includes aplurality of layers including the main electrode layer.
 14. The acousticwave device according to claim 11, wherein the main electrode layerincludes any one of Au, Pt, Ta, Cu, Ni, and Mo as a main component. 15.A high-frequency front end circuit comprising: the acoustic wave deviceaccording to claim 1; and a power amplifier.
 16. The high-frequencyfront end circuit according to claim 15, wherein the IDT electrodeincludes a plurality of layers including the main electrode layer.
 17. Ahigh-frequency front end circuit comprising: the acoustic wave deviceaccording to claim 11; and a power amplifier.
 18. The high-frequencyfront end circuit according to claim 17, wherein the IDT electrodeincludes a plurality of layers including the main electrode layer.
 19. Acommunication device, comprising: the high-frequency front end circuitaccording to claim 15; and an RF signal processing circuit.
 20. Acommunication device, comprising: the high-frequency front end circuitaccording to claim 17; and an RF signal processing circuit.